A known conventional motor driving device for driving a motor having mutually independent coil sets includes a plurality of energization systems each including a drive circuit for driving the corresponding one of the coil sets.
For example, the motor driving device for use in an electric power steering apparatus disclosed in Patent Document 1 includes such energization systems and power supply relays for the energization systems in a one-to-one correspondence. Each power supply relay, which is disposed between the power supply battery and the corresponding drive circuit, has two series-connected semiconductor switches. When an abnormality occurs in any of the energization systems, fail-safe processing is performed so as to turn off the power supply relay for this abnormal energization system and cut off power supply to the drive circuit located downstream to the power supply relay.
In each of the power supply relays for the energization systems provided in the motor driving device as described above, the two semiconductor switches are connected such that the forward directions of their parasitic diodes are opposite to each other. This aims to prevent formation of a closed circuit and resultant generation of a short circuit current while the power supply relays are turned off, which is otherwise expected when the power supply battery is connected to the motor driving device with reverse polarity by accident.